Uses of Custom Button and Link in Salesforce Lightning Component.

Hello Anjali,

Custom buttons and links help you integrate Salesforce data with external URLs, applications, your company’s intranet, or other back-end office systems.

Uses - 

1. Custom links can link to an external URL, such as www.google.com, a Visualforce page, or your company’s intranet.
2. Custom links can also link to a custom s-control in the custom s-control library, such as a Java applet or Active-X control.
3. Custom buttons can connect users to external applications, such as a web page that displays a map to a contact’s address.
4. Custom Button can run an s-control from the s-control library, such as an s-control that escalates a case from the case detail page.

Hope this will help you.

Thanks.